Known methods of oxygenating sewage or other aqueous waste material include that disclosed in GB-A-1 455 567 which discloses a process for treating an aqueous liquid, including the steps of taking a stream of the liquid, pressurizing the stream, introducing a treatment gas into the pressurized stream under turbulent conditions so as to dissolve some of the gas therein, and introducing the stream containing dissolved and undissolved gas through a nozzle into a volume of the liquid such that the undissolved gas enters the volume of liquid in the form of fine bubbles that become widely dispersed within the liquid and dissolve therein. By using this stream as a carrier of gas bubbles, a much greater quantity of gas can be successfully carried by the stream into the main volume of liquid and dissolved therein than if the amount of gas carried in the stream is merely limited to that which can be dissolved in the stream. Nonetheless, there is some loss of efficiency as a result of coalescence of the bubbles, particularly in the main volume of liquid.
Coalescence of bubbles reduces mass transfer from the gas phase to the liquid phase and results in undissolved gas rising to the surface of the volume of liquid and becoming vented to atmosphere as a waste gas stream. It is known that surfactants affect the dissolution of oxygen in water. For example, a paper entitled "Oxygen Transfer in Activated Sludge Basins of Sewage Treatment Plants", by H. H. Daucher, Ger. Chem. Eng., 1, (1978), pp 282-289, teaches that the oxygen transfer coefficient decreases in the presence of surfactants in jet aeration apparatus. Surprisingly, however, we have now found that selected surfactants can dramatically enhance gas dissolution in aqueous liquid.